Reactivity - Groups 1 & 2 (Edexcel International AS Chemistry): Revision Note
Reactivity Trend
Group 1 metals
The reactivity of the group 1 metals increases as you go down the group
When a group 1 element reacts its atoms only need to lose electron, as there is only 1 electron in the outer shell
When this happens, 1+ ions are formed
The next shell down automatically becomes the outermost shell and since it is already full, a group 1 ion obtains noble gas configuration
As you go down group 1, the number of shells of electrons increases by 1
This means that the outermost electron gets further away from the nucleus, so there are weaker forces of attraction between the outermost electron and the nucleus
Less energy is required to overcome the force of attraction as it gets weaker, so the outer electron is lost more easily
So, the alkali metals get more reactive as you descend the group
Group 2 metals
The reactivity of the Group 2 metals also increases down the group for the same reasons
The outermost electron gets further away from the nucleus, so there are weaker forces of attraction between the outermost electron and the nucleus
Less energy is required to overcome the force of attraction as it gets weaker, so the outer electron is lost more easily
This can be observed when the Group 2 metals react with water as well
Magnesium reacts extremely slowly with cold water
Calcium reacts fairly vigorously with cold water in an exothermic reaction
Reactions with Oxygen, Chlorine & Water
Group 1 metals
Reaction with oxygen
The alkali metals react with oxygen in the air forming metal oxides, which is why the alkali metals tarnish when exposed to the air
The metal oxide produced is a dull coating which covers the surface of the metal
The metals tarnish more rapidly as you go down the group
Summary of the Reactions of the First Three Alkali Metals with Oxygen
Reaction with chlorine
The reactions of the Group 1 elementrs with chlorine are similar in appearance to the reactions of the Group 1 metals with oxygen
Sodium, for example, burns with an intense orange flame in chlorine in exactly the same way that it does in pure oxygen
The other metals also behave the same way in both gases
In each case, a white solid is formed, the simple chloride
2M (s) + Cl2 (g) → 2MCl (s)
Reactions with water
The reactions of the alkali metals with water get more vigorous as you descend the group
Summary of the Reactions of the First Three Alkali Metals with Water
Group 2 metals
Reactions with water and oxygen
The reaction of Group 2 metals with oxygen follows the following general equation:
2M (s) + O2 (g) → 2MO (s)
Where M is any metal in Group 2
Remember than Sr and Ba also form a peroxide, MO2
The reaction of all metals with water follows the following general equation:
M (s) + 2H2O (l) → M(OH)2 (s) + H2 (g)
Except for, Be which does not react with water
Group 2 Metals Reacting with Water and Oxygen - Equations
| Reaction with oxygen | Reaction with water |
|---|---|---|
Mg | 2Mg (s) + O2 (g) → 2MgO (s) | Mg (s) + H2O (g) → MgO (s) + H2 (g) |
Ca | 2 Ca (s) + O2 (g) → 2CaO (s) | Ca (s) + 2H2O (l) → Ca(OH)2 (aq) + H2 (g) |
Sr | 2Sr (s) + O2 (g) → 2SrO (s) Sr (s) + O2 (g) → SrO2 (s) | Sr (s) + 2H2O (l) → Sr(OH)2 (aq) + H2 (g) |
Ba | 2Ba (s) + O2 (g) → 2BaO (s) Ba (s) + O2 (g) → BaO2 (s) | Ba (s) + 2H2O (l) → Ba(OH)2 (aq) + H2 (g) |
Magnesium reacts extremely slowly with cold water:
Mg (s) + 2H2O (l) → Mg(OH)2 (aq) + H2 (g)
The solution formed is weakly alkaline (pH 9-10) as magnesium hydroxide is only slightly soluble
However, when magnesium is heated in steam, it reacts vigorously with steam to make magnesium oxide and hydrogen gas:
Mg (s) + H2O (g) → MgO (s) + H2 (g)
Reactions with chlorine
Group 2 metals react with chlorine gas to give the metal chloride
For example
Mg (s) + Cl2 (g) → MgCl2 (s)
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